1. Field of the Invention
The present invention relates to a method and an apparatus for the determination of a condition or state of an object based on quasi-elastic interaction between the object and light transmitted to the object where light is transmitted to the object from a light source through an optical structure and light that has interacted with the object is collected and detected.
Prior art methods and apparatus of the above kind are based on the use of various conventional refractive elements such as beam splitters, refractive lenses and prisms, and birefringent elements, and suffer from a number of disadvantages.
One of the disadvantages is that the conventional refractive elements refracting the electromagnetic radiation are bulky and often difficult to adjust, and since a relatively large number of elements is required, the prior art apparatus of the above kind is relatively large, and expensive to manufacture.
Another disadvantage is that, since the refractive properties of conventional refractive elements, particularly birefringent elements, depend on the wavelength of the applied electromagnetic radiation, the determination is dependent on the wavelength. Therefore, it is common to use light sources with a highly stable wavelength, such as gas lasers, in a measurement apparatus of the above kind.
Yet another disadvantage is that use of bulky conventional refractive elements for beam splitting usually leads to unequal path lengths for the splitted beams, the difference often being in the order of centimeters, which makes it necessary to use light sources with a long coherence length, such as gas lasers, when coherent detection is needed.
Still another disadvantage is that since the refractive properties of conventional refractive elements are sensitive to ambient conditions, e.g. temperature, humidity, vibrations, etc., the determination is dependent on ambient conditions.
The use of gas lasers in a measurement apparatus of the above kind leads to a number of disadvantages. Firstly, gas lasers are bulky and expensive and require large, bulky, and expensive power supplies. Secondly, gas laser oscillates in a number of axial modes characterized in that the number of wavelengths between the two mirrors of the laser cavity is an integer. For example a HeNe-laser of a modest price typically oscillates in 5 modes while an Argon-laser may oscillate in several hundred modes. Typically the distance between different modes is around 100 Mhz. However, mixing between different modes caused by the non-linear laser medium can lead to intermediate frequencies in the range from 0 Hz to 100 Khz and very often these frequencies create signals in the measurement apparatuses of the above kind that can not be distinguished from the measurement signals of the apparatus and therefore may lead to misinterpretations.
Further, the emitted energy of the laser can shift between different modes so that the light intensity at different wavelengths varies up and down and in some instances a 100% modulation of the laser light has been seen leading to severe signal variations in the measurement apparatus.
2. Description of Prior Art
A number of different methods and apparatuses for the determination of a condition or state of an object based on quasi-elastic interaction between the object and light transmitted to the object are known in the art.
R. Schodl, Paper No. 21, AGARD Conference Proceedings No. 193, 1976 discloses a time-of-flight laser-dual-focus flow velocimeter comprising means to split a laser beam into two laser beams, means of focusing said beams in two small spots in a measuring volume, and detector means of producing double-pulses in a single detector from scattered light of single particles passing the two focused beam spots in the focal plane.
L. Lading, Paper No. 23, AGARD conference proceedings No. 193, discloses a time-of-flight laser anemometer comprising means to split a laser beam into two angular beams of orthogonal polarization, means for expanding and focusing the beam into two focal spots in the measuring volume, means for collecting light and imaging enlarged scatterings from the focal spots at two pinholes in front of two detectors, and means for cross-correlating signals of the two detectors.
Both these prior art techniques are based on conventional refractive and birefringent elements.
Steen Gruner Hanson: "Laser-based Method for Analysing Rotational Speed and Vorticity", International Symposium on Laser Anemometry, FED-Vol. 33, 1985, pp 91-95, discloses a method and an apparatus for the determination of rotational speed of solid bodies or of vorticity of fluids. A holographic optical element is used to diffract two light beams onto two spots on the object. The rotation of the object causes the light from the two spots scattered by the surface of the object to be Doppler shifted. The difference between the Doppler shifts of the light scattered from the two spots on the object indicates the rotational speed of the object.
Lars Lading et al.: "Analysis of a surface-scattering spectrometer", Journal of the Optical Society of America, Vol. 6, No. 11, Nov. 1989, discloses an apparatus for measuring the spatial and temporal statistics of surface fluctuations, in particular, thermally excited capillary waves on liquid surfaces.
In DE 42 40 735 A1, an optical device for determination of surface velocity of a moving object, e.g., a bar code during scanning, is disclosed. The device comprises a diffractive optical element for focusing two beams of light incident upon it on two regions of the object. The distance between the two regions are independent of the distance between the device and the object but it is wavelength dependent.
In EP 0 401 654 circular gratings for coupling of light into a waveguide structure are described.